U.S. patent number 7,032,135 [Application Number 10/211,065] was granted by the patent office on 2006-04-18 for equipment protection using a partial star architecture.
This patent grant is currently assigned to Corrigent Systems Ltd.. Invention is credited to Leon Bruckman, Rafi Harel, Gal Mor.
United States Patent |
7,032,135 |
Harel , et al. |
April 18, 2006 |
Equipment protection using a partial star architecture
Abstract
Communication apparatus includes a plurality of interface cards,
including a central interface card and spoke interface cards, which
are adapted to link communication lines to a network. A protection
bus includes multiple spoke connections that link the central
interface card to the spoke interface cards in a partial star
configuration, such that on at least one of the spoke connections
there are two of the spoke interface cards connected together to
the central interface card.
Inventors: |
Harel; Rafi (Beit Hashmonai,
IL), Bruckman; Leon (Petah Tikva, IL), Mor;
Gal (Herzlia, IL) |
Assignee: |
Corrigent Systems Ltd. (Tel
Aviv, IL)
|
Family
ID: |
32092282 |
Appl.
No.: |
10/211,065 |
Filed: |
August 2, 2002 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20040078620 A1 |
Apr 22, 2004 |
|
Current U.S.
Class: |
714/43; 370/425;
398/61; 709/252; 714/4.11; 714/E11.084 |
Current CPC
Class: |
G06F
11/2005 (20130101); H04Q 1/028 (20130101); H04Q
1/15 (20130101) |
Current International
Class: |
G06F
11/00 (20060101) |
Field of
Search: |
;398/63,61,43,4
;714/43,4,63,61 ;370/425,419 ;709/252 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cisco ONS 15454, available at:
http://www.cisco.com/warp/public/cc/pd/olpl/metro/on15454/prodlit/wldvs.s-
ub.--ds.htm, May 2002. cited by other .
"Star Network""Star Topology""Mesh Network", Microsoft Computer
Dictionary (Fifth Edition), 2002, Microsoft Press. cited by other
.
Null, Linda, The Essentials of Computer Organization and
Architecture 2003, Jones and Barlett Computer Science, p. 536.
cited by other.
|
Primary Examiner: Beausoliel; Robert
Assistant Examiner: McCarthy; Christopher
Attorney, Agent or Firm: Darby & Darby
Claims
The invention claimed is:
1. Communication apparatus, comprising: a plurality of interface
cards, including a central interface card and spoke interface
cards, which are adapted to link communication lines to a network;
and a protection bus comprising multiple spoke connections that
link the central interface card to the spoke interface cards in a
partial star configuration, such that on at least one of the spoke
connections there are two of the spoke interface cards connected
together to the central interface card, wherein no more than a
single one of the spoke connections has only one of the spoke
interface cards connected thereto, while the remaining spoke
connections have respective pairs of the spoke interface cards
connected thereto.
2. Apparatus according to claim 1, wherein each of the pairs of the
spoke interface cards comprises a working card and a protection
card, which are adapted to operate in a 1:1 protection
configuration.
3. Apparatus according to claim 2, wherein the working card
comprises a connection interface, which is connected to receive and
transmit signals over one of the communication lines, and an input
switch, which is operable so as to transfer the signals via one of
the spoke connections to the protection card when a fault occurs in
the working card.
4. Apparatus according to claim 3, wherein both the working card
and the protection card comprise processing circuitry, for
processing the signals, and wherein the protection card comprises
an output switch, which is operable so as to receive the signals
from the one of the spoke connections when the fault occurs in the
working card and to convey the signals to the processing circuitry
of the protection card for processing thereby.
5. Apparatus according to claim 2, wherein the central interface
card is adapted to operate in the 1:1 protection configuration in
conjunction with the one of the spoke interface cards that is
connected to the single one of the spoke connections having only
the one of the spoke interface cards connected thereto.
6. Apparatus according to claim 1, wherein the spoke interface
cards are adapted to serve as working cards, comprising respective
connection interfaces, which are connected to receive and transmit
signals over the communication lines, and wherein the central
interface card is adapted to serve as a protection card for the
working cards in a 1:N protection configuration.
7. Apparatus according to claim 6, wherein each of the spoke
interface card comprises an input switch, which is operable so as
to transfer the signals via one of the spoke connections to the
central interface card when a fault occurs in the spoke interface
card.
8. Apparatus according to claim 7, wherein both the spoke interface
card and the central interface card comprise processing circuitry,
for processing the signals, and wherein for each one of the spoke
connections, the central interface card comprises a respective
output switch, which is operable so as to receive the signals from
the one of the spoke connections when the fault occurs in the spoke
interface card and to convey the signals to the processing
circuitry of the central interface card for processing thereby.
9. Apparatus according to claim 6, wherein the interface cards
comprise switches for selecting the 1:N protection configuration or
a 1:1 protection configuration, in which each of the pairs of the
spoke interface cards comprises a working card and a protection
card.
10. A communication backplane, comprising: a backplane substrate; a
plurality of receptacles fixed to the backplane substrate for
receiving communication interface cards, the receptacles including
a central receptacle and spoke receptacles; and printed circuit
traces arranged on the backplane substrate to form a protection bus
that comprises multiple spoke connections linking the central
receptacle to the spoke receptacles in a partial star
configuration, such that on at least one of the spoke connections
there are two of the spoke receptacles connected together to the
central receptacle, wherein no more than a single one of the spoke
connections has only one of the spoke receptacles connected
thereto, while the remaining spoke connections have respective
pairs of the spoke receptacles connected thereto.
11. A method for fault protection in communication equipment,
comprising: arranging a plurality of interface cards, including a
central interface card and spoke interface cards, to link
communication lines to a network; interconnecting the interface
cards with a protection bus comprising multiple spoke connections
that link the central interface card to the spoke interface cards
in a partial star configuration, such that on at least one of the
spoke connections there are two of the spoke interface cards
connected together to the central interface card; determining that
a fault has occurred in a first one of the interface cards serving
a first one of the communication lines; and responsive to the
fault, conveying signals over the protection bus from the first one
of the interface cards to a second one of the interface cards, so
that the second one of the interface cards serves the first one of
the communication lines in place of the first one of the interface
cards, wherein interconnecting the interface cards comprises making
the spoke connections so that no more than a single one of the
spoke connections has only one of the spoke interface cards
connected thereto, while the remaining spoke connections have
respective pairs of the spoke interface cards connected
thereto.
12. A method according to claim 11, wherein making the spoke
connections comprises connecting each of the pairs of the spoke
interface cards to operate as a working card and a protection card
in a 1:1 protection configuration.
13. A method according to claim 12, wherein arranging the plurality
of interface cards comprises connecting the working card via a
connection interface to receive and transmit signals over one of
the communication lines, and wherein conveying the signals over the
protection bus comprises transferring the signals from the
connection interface via one of the spoke connections to the
protection card responsive to the fault in the working card.
14. A method according to claim 12, wherein making the spoke
connections comprises configuring the central interface card to
operate in the 1:1 protection configuration in conjunction with the
one of the spoke interface cards that is connected to the single
one of the spoke connections having only the one of the spoke
interface cards connected thereto.
15. A method according to claim 11, wherein arranging the plurality
of interface cards comprises connecting the spoke interface cards
to serve as working cards, which receive and transmit signals over
the communication lines via respective connection interfaces, and
wherein interconnecting the interface cards comprises connecting
the central interface card to serve as a protection card for the
working cards in a 1:N protection configuration.
16. A method according to claim 15, wherein conveying the signals
over the protection bus comprises transferring the signals via one
of the spoke connections to the central interface card responsive
to the fault in one of the spoke interface cards.
17. A method according to claim 15, and comprising switching
between the 1:N protection configuration and a 1:1 protection
configuration, in which each of the pairs of the spoke interface
cards comprises a working card and a protection card.
Description
FIELD OF THE INVENTION
The present invention relates generally to communication systems,
and specifically to protecting communication systems against
equipment failures.
BACKGROUND OF THE INVENTION
Equipment protection--providing reliable backup service in the case
of failure--is an essential part of most high-speed
telecommunication systems. Telecommunication equipment typically
includes some redundant components, which automatically take over
for faulty components when failure occurs. Common protection
schemes include 1:1 and 1:N systems. In a 1:1 system, each working
communication interface has a dedicated backup interface, also
referred to as a protection interface, which remains on standby as
long as the working interface is functioning properly. In a 1:N
system, a single backup interface serves N working interfaces. The
choice of N depends on a tradeoff between cost and reliability
demands.
Backplane-based configurations are commonly used in communication
and computing equipment. In network access systems, for example, a
backplane may be used to connect a main module, having a trunk link
to a core network, to a number of subsidiary modules having ports
such as DS-3 interfaces, which serve network users (The DS-3 level
of the plesiochronous digital hierarchy [PDH] is used in
circuit-switched communication networks to carry medium-speed
traffic at 44.736 Mbps.) The main and subsidiary modules comprise
interface cards, also referred to as line cards, with plug into
suitable receptacles, typically edge connectors, on the backplane.
Printed circuit traces on the backplane connect the subsidiary
modules to the ports of the main module, as well as passing
different types of signals (such as data and clock signals) between
the line cards located in the same chassis.
Equipment protection in such backplane-based systems typically
requires that redundant interface cards be installed in the chassis
(also referred to as a shelf). Data signals are conveyed between
the redundant cards and the working interface cards using either a
box external to the chassis or a protection bus inside the chassis.
To avoid awkward and potentially unreliable cabling, the protection
bus is best implemented using traces on the backplane. The choice
of whether to use 1:1 or 1:N protection is usually made by the
network operator based on the cost and reliability constraints of
the particular application environment. These constraints may
change over time, or when existing equipment is redeployed in a new
location or application.
In response to the need for flexible protection configuration, some
systems offer the network operator the possibility of configuring
the equipment in either a 1:1 or 1:N protection topology.
Typically, separate traces are provided on the backplane for the
1:1 and 1:N protection buses, meaning that additional printed
circuit traces must be introduced on a board that is already
crowded with conductors carrying high-speed signals. Each edge
connector on the backplane must also have additional pins to
accommodate the different protection buses, and a relay or other
switch must be added for selecting the protection mode to be used.
It can thus be appreciated that this implementation approach has a
number of disadvantages in terms of equipment cost and
complexity.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to reduce
the hardware burden and complexity associated with providing
flexible protection configuration, particularly in backplane-based
equipment.
In preferred embodiments of the present invention, communication
equipment comprises a plurality of interface cards, interconnected
by a protection bus having a partial star topology. In this
topology, one of the interface cards, referred to as the central
interface card, is connected at the hub of the star, while the
remaining cards are connected to the spokes, with two of the
interface cards connected together on each of one or more of the
spokes. Each of the interface cards comprises one or more switches
for selecting the protection configuration of the equipment. For
1:N configuration, the switches are set so that the central
interface card serves as the protection card for all the other
interface cards in the partial star. For 1:1 configuration, the
switches are set so that the two interface cards connected together
on each spoke operate as a 1:1 protection pair (possibly except for
one of the spoke interface cards that is paired with the central
interface card).
Thus, the single partial-star protection bus is configurable for
both 1:1 and 1:N protection, with the same spokes of the bus
serving both configurations. In backplane-based systems, the use of
such a bus reduces the number of traces required on the backplane
and the number of pins required in each edge connector. It also
reduces the number of interfaces that the central interface card
must have in order to provide 1:N protection, since some of the
spokes (typically all the spokes, or all but one of the spokes) are
used to link two interface cards to the central interface card.
Although the preferred embodiments described herein are directed to
backplane-based systems that are configurable for 1:1 and 1:N
protection, the partial star architecture may also be used in other
flexible protection schemes.
There is therefore provided, in accordance with a preferred
embodiment of the present invention, communication apparatus,
including:
a plurality of interface cards, including a central interface card
and spoke interface cards, which are adapted to link communication
lines to a network; and
a protection bus including multiple spoke connections that link the
central interface card to the spoke interface cards in a partial
star configuration, such that on at least one of the spoke
connections there are two of the spoke interface cards connected
together to the central interface card.
Preferably, no more than a single one of the spoke connections has
only one of the spoke interface cards connected thereto, while the
remaining spoke connections have respective pairs of the spoke
interface cards connected thereto. Further preferably, each of the
pairs of the spoke interface cards includes a working card and a
protection card, which are adapted to operate in a 1:1 protection
configuration. Most preferably, the working card includes a
connection interface, which is connected to receive and transmit
signals over one of the communication lines, and an input switch,
which is operable so as to transfer the signals via one of the
spoke connections to the protection card when a fault occurs in the
working card. In addition both the working card and the protection
card preferably include processing circuitry, for processing the
signals, and the protection card includes an output switch, which
is operable so as to receive the signals from the one of the spoke
connections when the fault occurs in the working card and to convey
the signals to the processing circuitry of the protection card for
processing thereby.
In a preferred embodiment, the central interface card is adapted to
operate in the 1:1 protection configuration in conjunction with the
one of the spoke interface cards that is connected to the single
one of the spoke connections having only the one of the spoke
interface cards connected thereto.
Preferably, the spoke interface cards are adapted to serve as
working cards, including respective connection interfaces, which
are connected to receive and transmit signals over the
communication lines, and the central interface card is adapted to
serve as a protection card for the working cards in a 1:N
protection configuration. Further preferably, each of the spoke
interface card includes an input switch, which is operable so as to
transfer the signals via one of the spoke connections to the
central interface card when a fault occurs in the spoke interface
card. Most preferably, both the spoke interface card and the
central interface card include processing circuitry, for processing
the signals, and for each one of the spoke connections, the central
interface card includes a respective output switch, which is
operable so as to receive the signals from the one of the spoke
connections when the fault occurs in the spoke interface card and
to convey the signals to the processing circuitry of the central
interface card for processing thereby.
There is also provided, in accordance with a preferred embodiment
of the present invention, a communication backplane, including:
a backplane substrate;
a plurality of receptacles fixed to the backplane substrate for
receiving communication interface cards, the receptacles including
a central receptacle and spoke receptacles; and
printed circuit traces arranged on the backplane substrate to form
a protection bus that includes multiple spoke connections linking
the central receptacle to the spoke receptacles in a partial star
configuration, such that on at least one of the spoke connections
there are two of the spoke receptacles connected together to the
central receptacle.
Preferably, no more than a single one of the spoke connections has
only one of the spoke receptacles connected thereto, while the
remaining spoke connections have respective pairs of the spoke
receptacles connected thereto.
There is additionally provided, in accordance with a preferred
embodiment of the present invention, a method for fault protection
in communication equipment, including:
arranging a plurality of interface cards, including a central
interface card and spoke interface cards, to link communication
lines to a network;
interconnecting the interface cards with a protection bus including
multiple spoke connections that link the central interface card to
the spoke interface cards in a partial star configuration, such
that on at least one of the spoke connections there are two of the
spoke interface cards connected together to the central interface
card;
determining that a fault has occurred in a first one of the
interface cards serving a first one of the communication lines;
and
responsive to the fault, conveying signals over the protection bus
from the first one of the interface cards to a second one of the
interface cards, so that the second one of the interface cards
serves the first one of the communication lines in place of the
first one of the interface cards.
The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram that schematically illustrates
communication equipment with a partial-star protection bus, in
accordance with a preferred embodiment of the present
invention;
FIG. 2 is a block diagram that schematically illustrates an
interface card for use in the equipment of FIG. 1, in accordance
with a preferred embodiment of the present invention;
FIG. 3 is a block diagram that schematically illustrates a 1:1 pair
of interface cards in a normal working configuration, in accordance
with a preferred embodiment of the present invention;
FIG. 4 is a block diagram that schematically illustrates the pair
of interface cards of FIG. 3 in a protection configuration;
FIG. 5 is a block diagram that schematically illustrates a central
interface card for use in the equipment of FIG. 1, in accordance
with a preferred embodiment of the present invention;
FIG. 6 is a block diagram that schematically illustrates a number
of interface cards together with the central interface card of FIG.
5 in a 1:N working configuration, in accordance with a preferred
embodiment of the present invention; and
FIG. 7 is a block diagram that schematically illustrates the
interface cards of FIG. 6 in a protection configuration.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is a block diagram that schematically illustrates
communication equipment 20, in accordance with a preferred
embodiment of the present invention. Equipment 20 comprises a
backplane 22, to which multiple interface cards are connected. The
interface cards include a main module 24 and six subsidiary modules
26, 28, also referred to hereinafter as line cards (labeled LC1
through LC6). The main and subsidiary modules plug into receptacles
25, typically edge connectors, or slots, on backplane 22. The
backplane comprises a printed circuit substrate, with printed
circuit traces 30 formed thereon so as to interconnect receptacles
25. Although in this embodiment, equipment 20 comprises six
subsidiary modules, greater or smaller numbers of modules may
similarly be used.
Main module 24 typically comprises a network interface, which
connects equipment 20 to a core network, along with a switch (not
shown) for multiplexing among the subsidiary modules. Each of
subsidiary modules 26, 28 has a user line interface that is coupled
to a respective connector 32 on a connection panel 34. The user
line interfaces may comprise substantially any suitable types of
interfaces known in the art. For example, the user line interfaces
may comprise DS3 interfaces. In this case, a respective DS3 user
line can then be connected to each of connectors 32, enabling
incoming signals from the user line to be transmitted onto the core
network, and outgoing signals from the network to be conveyed to
the user line.
Subsidiary modules 26, 28 comprise five spoke line cards 26 and one
central line card 28, which are mutually linked by a protection bus
36. The protection bus has a partial star topology, meaning that
there are multiple spoke line cards connected to each of one or
more of the spokes of the star. Because equipment 20 is designed to
support 1:N and 1:1 protection configurations, each spoke has
either one or two spoke cards connected thereto. In the present
embodiment, LC1 and LC2 are together connected to one spoke, while
LC5 and LC6 are connected to another. LC3 is connected by its own
spoke to the central card LC4. The number of spokes is thus equal
to ceil(N/2), i.e., the smallest integer that is no less than
N/2--three spokes in the present example. In the 1:N configuration
(in this case, N=5), LC4 may serve as the protection card for all
of spoke cards 26 or for a subset of the spoke cards. In the 1:1
configuration, there are three 1:1 protection pairs: LC1 LC2, LC3
LC4 and LC5 LC6. Bus 36, which preferably comprises a set of traces
on backplane 22, serves both the 1:N and 1:1 configurations. The
same traces are used for both configurations, depending on the
setting of switches on the line cards, as described
hereinbelow.
FIG. 2 is a block diagram that schematically illustrates one of
spoke line cards 26, in accordance with a preferred embodiment of
the present invention. The spoke line card receives and processes
incoming signals from its respective user line via connector 32 and
conveys the processed signals to main module 24. In the normal
working configuration shown here, an input switch 40 connects the
user line to an output switch 42, which in turn connects to signal
processing circuitry 44. Circuitry 44 processes the incoming user
line signals, as is known in the art, following which the processed
signals are conveyed through traces 30 to main module 24. An
embedded controller 46 controls the settings of switches 40 and 42,
depending on whether card 26 is in its normal working mode or
protection mode. The design and implementation of processing
circuitry 44 and controller 46 are straightforward, and various
possible designs will be apparent to those skilled in the art.
Theses elements are omitted from subsequent figures for the sake of
simplicity.
Outgoing signals from main module 24 are similarly conveyed via
traces 30 to circuitry 44 on line card 26. In the normal working
configuration, the outgoing signals are switched to connector 32 in
the same manner as are the incoming signals shown in FIG. 2. The
additional lines and switch contacts required for this purpose on
card 26 are omitted from the figures for the sake of simplicity.
Generally speaking, the switching and circuitry required for both
normal operation and protection of line cards 26 and 28 in
processing outgoing signals is simply the mirror image of that
required for incoming signals.
FIG. 3 is a block diagram that schematically illustrates spoke line
cards 26a and 26b in a 1:1 protection configuration, during normal
working operation. Card 26a could be line card LC1, as shown in
FIG. 1, which card 26b is LC2. The same type of line card may be
used for both the working card 26a and the standby protection card
26b. (Lower-case suffixes in reference numbers, such as the
suffixes "a" and "b," are used here simply to indicate different
cards and components of the same types. Alternatively, cards 26a
and 26b could represent any one of the other 1:1 pairs noted
above.) Input switch 40a of working card 26a is connected to
connector 32, while input switch 40b of the protection card is
unconnected. As long as working card 26a is functioning properly,
all signals between the user line and main module 24 are processed
by card 26a, with switches 40a and 42a held in the configuration
shown in FIG. 3.
FIG. 4 is a block diagram that schematically illustrates protection
operation of spoke line cards 26a and 26b. Here it is assumed that
a fault has been detected in working card 26a or in a trace 30 that
connects the working card to main module 24. As a result, input
switch 40a is toggled to connect the user line at connector 32 to
protection bus 36. At the same time, output switch 42b of
protection card 26b is toggled to capture the incoming signals from
the protection bus. These signals are now processed by the
protection card and conveyed via traces 30 to the main module.
Outgoing signals are processed and switched in like manner.
FIG. 5 is a block diagram that schematically illustrates central
line card 28, in accordance with a preferred embodiment of the
present invention. A respective spoke of protection bus 36 connects
central line card 28 to each pair of spoke cards 26, as shown in
FIG. 1. For each spoke that is connected to it, the central line
card has a respective input switch 50, 52, which is used to select
the spoke for which protection is required. In the switch
configuration shown in FIG. 5, all the spoke line cards are assumed
to be working normally, so that switches 50 and 52 are in their
unconnected positions. Since each input switch 50, 52 serves two
spoke line cards, the number of switches needed on central line
card 28 is substantially smaller than the number that would be
required to implement a full-star topology. The central line card
also comprises signal processing circuitry 44 and an embedded
controller 46, as described above.
A further spoke of protection bus 36 connects to the single spoke
card (LC3 in FIG. 1) for which central card 28 provides both 1:1
and 1:N protection. In the embodiment shown here, the central card
also has an optional connector 32 and input and output switches 40
and 42, like the spoke cards. These elements enable the central
card to operate as a spoke card if desired, in addition to or
alternatively to its central protection function. Alternatively, if
the central card is to serve only for protection purposes,
connector 32 and input switch 40 may be eliminated. Further
alternatively, if the central card is to provide only 1:N
protection (as for example, when there is an odd number of line
cards on the protection bus), each spoke of the protection bus may
have two spoke line cards attached to it.
FIG. 6 is a block diagram that schematically illustrates central
line card 28 and spoke line cards 26a 26d in a 1:N protection
configuration, during normal working operation. Cards 26a and 26b
are connected together to one of the spokes, while cards 26c and
26d are connected to another, in the manner shown in FIG. 1. Input
switches 40a and 40c and output switches 42a and 42c (and likewise
the switches on cards 26b and 26d, which are not shown) are
configured so that each of the spoke cards processes the incoming
signals from and outgoing signals to its own user line. The same
traces of protection bus 36 that were used to connect the 1:1
pairs, such as cards 26a and 26b, in the 1:1 protection
configuration of FIGS. 3 and 4 serve as the spokes connecting the
spoke line cards to central line card 28 in the 1:N configuration
of FIG. 6. As long as all the spoke line cards are working
properly, central line card 28 remains on standby.
FIG. 7 is a block diagram that schematically illustrates protection
operation of central line card 28. A fault is assumed to have
occurred in spoke line card 26c, as shown in the figure. Input
switch 40c is therefore toggled to convey signals via protection
bus 36 to central line card 28. Input switch 52 of the central line
card is similarly toggled to receive and process the signals, in
place of the faulty spoke line card. The remaining spoke line cards
continue working normally. Of course, if two line cards fail
simultaneously, central line card 28 will be able to protect only
one of them, but this is an inherent feature of 1:N protection
schemes.
Although the preferred embodiments described hereinabove are
directed to backplane-based access equipment 20, the partial-star
protection architecture may similarly be used in other types of
communication platforms, used both for access and core network
functions. Furthermore, while protection bus 36 used in equipment
20 is configurable for 1:1 and 1:N protection, the partial star
architecture may alternatively be used in other flexible protection
schemes. For example, some or all of the spokes in the partial star
may have M+1 interface cards connected thereto (M>1). The
interface cards on such spokes will then be configurable for either
1:N or 1:M protection. Alternative protection topologies based on
the principles of the present invention will be apparent to those
skilled in the art.
It will thus be appreciated that the preferred embodiments
described above are cited by way of example, and that the present
invention is not limited to what has been particularly shown and
described hereinabove. Rather, the scope of the present invention
includes both combinations and subcombinations of the various
features described hereinabove, as well as variations and
modifications thereof which would occur to persons skilled in the
art upon reading the foregoing description and which are not
disclosed in the prior art.
* * * * *
References